The extraction process which comprises contacting the feedstock to be treated, in liquid phase, with a third, countercurrently circulating solvent, also in the liquid phase. This is a well-known industrial operation and is intended to ensure the transfer of one or more feedstock solutes to an immiscible phase, namely the third liquid solvent. This results in two phases: an extract phase (solvent and solute) and a raffinate phase (feedstock freed of a major part of solute).
Such a process is used, in particular, to eliminate the aromatic compounds present in feedstocks used to formulate lubricant oils. This is because said oils must show only minor variations in viscosity with temperature, and the aromatic compounds show highly variable viscosity. Moreover, the aromatic compounds are sensitive to oxidation and can cause coke formation by polycondensation.
In general, the solvent and the feedstock to be treated are made to circulate countercurrently in an extraction column. The solvent which is heavier than the feedstock is introduced at the upper part, and the feedstock at the lower part, of the column. A raffinate phase, impoverished in aromatic compounds stemming from the original feedstock, and small quantities of solvent are collected at the top, and an extract phase containing a major part of the solvent and the dissolved aromatic compounds stemming from the original feedstock is collected at the bottom of the column.